2,5-Furandicarboxylic acid (FDCA) is a diacid that can be produced from natural sources such as carbohydrates. Routes for its preparation using air oxidation of 2,5-disubstituted furans such as 5-hydroxymethylfurfural or ethers thereof with catalysts comprising Co and Mn have been disclosed in e.g. WO2010/132740, WO2011/043660 and WO2011/043661.
U.S. Pat. No. 2,551,731 describes the preparation of polyesters and polyester-amides by reacting glycols with dicarboxylic acids of which at least one contains a heterocyclic ring, such as 2,5-FDCA. Under melt polymerization conditions, using sodium- and magnesium methoxide as a catalyst, FDCA and 2.5 equivalents of ethylene glycol or FDCA dimethyl ester and 1.6 equivalents of ethylene glycol were reacted in a esterification step or transesterification step, respectively, at ambient pressure between 160 and 220° C., after which a polycondensation was carried out between 190 and 220° C. under a few mm Hg pressure. The polycondensation process took between about 5 to over 7 hours. The product had a reported melting point of 205-210° C. and readily yielded filaments from the melt.
In US 2009/0124763 polyesters are described, having a 2,5-furandicarboxylate moiety within the polymer backbone and having a degree of polymerization of 185 or more and 600 or less. These polymers are made in a three step process involving the esterification of the 2,5-FDCA or the transesterification of the diester thereof with a diol, and a second step involving polycondensation, followed by solid state polymerization as third step.
The first step is carried out at ambient pressure at a temperature within a range of 150 to 180° C., whereas the polycondensation step is carried out under vacuum at a temperature within a range of 180 to 230° C. The product is then purified by dissolving the same in hexafluoroisopropanol, re-precipitation and drying, followed by the third step, a solid state polymerization at a temperature in the range of from 140 to 180° C. For the preparation of poly(ethylene furan dicarboxylate) the first two steps took over 11 hours.
In WO 2010/077133 a process for preparing furandicarboxylate-containing polyesters is described wherein the diester of FDCA is transesterified with a diol, and the ester composition thus obtained is subjected to polycondensation. The polycondensation is conducted for a period of up to 5 hours. The polycondensate may then be subjected to solid state polymerization. In an example the solid state polymerization was conducted for 60 hours. Although the molecular weight of the polyester obtained is reasonably high, the duration of the solid state polymerization is considered too long. An improvement is described in WO 2013/062408, wherein the dimethyl ester of FDCA is transesterified with ethylene glycol, or bis(2-hydroxyethyl)-2,5-furandicarboxylate is used as starting material. The transesterification product or this starting material is then subjected to polycondensation and after a drying/crystallization step the polycondensate is subjected to solid state polymerization. The polycondensation was shown to take three hours. In an example the solid state polymerization takes two days.
In WO 2013/120989 a continuous process for the preparation of poly(ethylene furandicarboxylate) is described wherein FDCA or a diester thereof is mixed with ethylene glycol at elevated temperature to give a paste or a homogeneous solution, the paste or solution is converted to an esterification product of FDCA and ethylene glycol, the esterification product is polycondensed under reduced pressure, wherein the polycondensation is performed in two stages. According to an example the dimethyl ester of FDCA was reacted with ethylene glycol in a molar ratio of 1:1.7. In this example the stages following the production of the esterified product took 5 hours. The polycondensation product can be subjected, if desired, to a solid stating polymerization.
US 2014/0024793 discloses a process for the production of a polyester comprising an esterification reaction between ethylene glycol and FDCA to form an oligomer, and a condensation reaction of the oligomer obtained, wherein the esterification reaction is carried out at a temperature of 200 to 250° C. According to the formulae in the application, all oligomers and polyesters have hydroxyl end groups. In an example the molar ratio of ethylene glycol to FDCA is 2:1 and the esterification temperature is 230° C.
KR 20140003167 describes a polyester polymer with excellent transparency which is manufactured by using a biomass originated furandicarboxylate ester compound with ethylene glycol. In comparative examples also furandicarboxylic acid has been used. The molar ratio of furandicarboxylate ester to ethylene glycol may be from 1:1.1 to 1:4. The ratio of furandicarboxylic acid to ethylene glycol varies between 1:1.2 to 1:2.
US 2013/171397 describes the preparation of copolyesters formed from 2,5-furandicarboxylic acid or a lower alkyl ester thereof, at least one aliphatic or cycloaliphatic C3-C10 diol and terephthalic acid. In a comparative experiment the patent application describes the polymerization of furandicarboxylic acid and ethylene glycol in a molar ratio of 1:1.5.
It has been found that the esterification reaction for 2,5-furandicarboxylic acid with ethylene glycol is quite fast. As a result it is most common to “over-esterify” and leave an inadequate amount of carboxylic acid end groups in the polyester. The inadequate amount of carboxylic end groups in the polyester may lead to relatively slow polycondensation to form the desired polyester. Further, if the polyester is subjected to solid state polymerization, the low level of carboxylic end groups may lead to a slow increase of the molecular weight. Moreover, the particle size of the polyester influences the rate at which the molecular weight increases. In order to arrive at an acceptable rate the particles must be uneconomically small. Therefore, there is a need for a method that ensures that the esterification reaction of 2,5-furandicarboxylic acid with ethylene glycol does not yield “over-esterified” polyester with an inadequate level of carboxylic end groups. It has now been found that the extent of the esterification reaction can be controlled by using a dimensionless parameter.